In a tape drive, such as linear tape drive, the tape speed during operation is typically around 5-10 meters per second, with the reels rotating at around 20-60 times per second. Additionally, on a typical half inch tape, there may be 1000 or more data tracks spaced laterally across the tape. Because of these speeds and the density of data tracks several characteristics related to the reels need to be well controlled, or else errors in reading and writing the tape are more likely to occur.
For example, the off-center run-out of the reels needs to be well controlled to avoid exciting tape vibration. It is also important to avoid any sudden protrusion or recession on the reel hubs that can cause sudden disturbance to the tape motion longitudinally and/or laterally. Longitudinal tape motion can cause an error in linear density of the data, while lateral tape motion can be so fast that the head of the tape drive cannot accurately follow and read from or write to the tightly spaced adjacent data tracks. At an extreme, a protrusion or recession on a tape reel can also cause embossment on the tape and possibly data loss.
In a single reel tape cartridge drive, the tape drive utilizes grabbers or bucklers to thread the tape from the cartridge to the drive reel. As such, there is an unavoidable grabber or buckler mismatch to the drive reel hub. Further, in a single reel cartridge, media in the cartridge usually employs a splicing tape to connect the leader tape to the magnetic tape. The added thickness of this splicing tape also adds mismatch to the drive reel hub.
In an effort to avoid the problems described above, reel hub mismatch is typically tightly controlled, often with 50 μm or less. A conventional method for control employs a dial indicator to measure assembled drive reel hubs to confirm that circularity falls within a predefined range. While this provides a good initial check, dial indicator testing lacks accuracy because pressure is not applied to the grabbers or bucklers as would be applied by a tape wrapped around the drive reel hub or cartridge reel hub in an operational situation. Further, use of a dial indicator in an operational situation would scratch a tape. Additionally, a dial indicator measurement would be difficult or impossible within an assembled tape drive.